1. Field of the Invention
The present invention relates to a magnetic random access memory (MRAM) device and an electronic card and an electronic device using such a memory device and more specifically to the structure of a memory cell comprised of a storage element that stores a “1” or “0” of data using a tunneling magnetoresistive effect.
2. Description of the Related Art
In recent years, there have been proposed many memory devices which store information based on a new principle. As one of such memory device, an MRAM device having both the non-volatility and the rapidity in which a plurality of memory cells including magnetic tunnel junction elements (which will be referred to as MTJ elements hereinafter) having a tunneling magnetoresistive effect are arranged in a matrix form is disclosed in, e.g., Roy Scheuerlein et. al. “A 10 ns Read and Write Non-Volatile Memory Array Using a Magnetic Tunnel Junction and FET Switch in each Cell”, ISSCC2000 Technical Digest pp. 128 to pp. 129.
The MTJ element has two magnetic layers which are generally referred to as a recording layer and a fixed layer. When programming data in the MTJ element, a current is caused to flow through a write wiring, and a magnetic field in a predetermined direction is applied to the MTJ element, thereby switching the direction of magnetization of the recording layer.
Meanwhile, the most serious problem in the MRAM device is a reduction in a write current. The present inventors found that overcoming a thermal agitation problem as a result of an experiment of holding the reliability of the MTJ element. This prehistory will now be described hereinafter.
Under the present situation, a write current value of the MTJ element is as large as 8 to 10 mA. For a practical application, the write current value must be lowered to an allowable level. In the case of a test chip of the MRAM device on a 1 K-bit level manufactured by the present inventors by way of trial, the write current value is 8 to 10 mA as was expected.
Further, bit information retention characteristics of the MTJ element were examined. As a result, irrespective of a fact that criteria Ku×V/kB×T of the thermal agitation property which are usually considered in a magnetic medium of a hard disk storage apparatus are set to be not less than 80, some bits were switched. Here, V is a cubic volume of a recording layer of the MTJ element, kB is the Boltzmann constant, and T is an absolute temperature. In case of the MRAM device, Ku is given mainly based on a shape magnetic anisotropy as a general rule, and it is actually a sum of an anisotropic energy and an induced magnetic anisotropy.
For improving the thermal agitation property in order to prevent the bit information from being switched, Ku×V is usually set large. By doing so, however, the write current is increased.
In the MRAM device, it is desirable to achieve both a reduction in the write current and overcoming the thermal agitation property as described above. In the prior art, however, a concrete design plan for this purpose is not proposed. The prehistory that this problem was found will now be described in detail hereinafter.
At present, a reported write current value of the MTJ element is at least approximately 8 mA if a width of the MTJ element is approximately 0.6 μm and a length of the MTJ element is approximately 1.2 μm.
Usually, a shape of the MTJ element is determined as a rectangular or an ellipse, the shape magnetic anisotropy is given to the MTJ element, a direction of magnetization of the MTJ element is stipulated, and the thermal agitation property is also improved.
Ku×V is a product of a sum of the shape magnetic anisotropy and the induced magnetic anisotropy of the MTJ element, and a volume of the recording layer of the MTJ element. Here, the induced magnetic anisotropy of the recording layer is given in the same direction as that of the anisotropy based on a shape so as not to generate the dispersion of the anisotropy or the like. However, usually, NiFe used as a material of the recording layer has the induced magnetic anisotropy (several Oe) smaller than the anisotropic magnetic field (several ten Oe) based on a shape by a single digit, and it is considered that the thermal agitation property and the switching magnetic field are also substantially determined by the shape magnetic anisotropy.
The switching magnetic field Hsw required to rewrite magnetization information of the recording layer is substantially given by the following expression (1).Hsw=4π×Ms×t/F(Oe)  (1)
Here, Ms is a saturation magnetization of the recording layer, t is a thickness of the recording layer, and F is a width of the recording layer. Further, a sum Ku of the anisotropic energy based on a shape and the induced magnetic anisotropy is substantially given by the following expression (2).Ku=Hsw×Ms/2  (2)
As a method for reducing the write current, coating a conventional write wiring made of, e.g., Cu with a soft magnetic material such as NiFe and using it as a write wiring with a yoke is proposed in, e.g., Saied Tehrani, “Magneto resistive RAM”, 2001 IEDM short course. According to this method, the approximately twofold high-efficiency effect, i.e., the write current value can be reduced to approximately ½.
FIG. 1 shows an example of a structure of the write wiring with a yoke described in the above cited reference (“Magneto resistive RAM”), and FIG. 2 shows a result of examining write characteristics obtained by using the write wiring illustrated in FIG. 1. As shown in FIG. 1, the write wiring with a yoke has a structure that a part of the periphery of a write wiring 30 made of Cu is coated with a yoke 20 made of a soft magnetic material such as NiFe.
In FIG. 2, characteristics A indicated by a solid line show a state that a width F of a recording layer is reduced and a switching magnetic field Hsw is increased as minuteness of an MTJ element is realized when a CoFeNi thin film having a film thickness of 2 nm is used as the recording layer.
In case of using the conventional write wiring (characteristics B), since the generated magnetic field is larger than the switching magnetic field until 1/F is approximately 7, writing is possible. On the other hand, in case of using the conventional write wiring with a yoke (characteristics C), since the generated magnetic field is larger than the switching magnetic field even if 1/F exceeds approximately 7, writing is possible, but the generated magnetic field is smaller than the switching magnetic field when 1/F exceeds approximately 10.
As a result of examining the case that write wiring with a yoke formed by a prior art is used based on an experiment and a computer simulation, the approximately twofold high-efficiency effect was confirmed, and the write current can be reduced to 5 mA. However, this is the limit, and it is far from 1 to 2 mA which is a target value required for a practical application.
Meanwhile, the measurements of the magnetoresistive effect when a GdFe alloy perpendicular magnetization film is used for a TMR film are reported by Ikeda et al., in “GMR and TMR films using GdFe alloy perpendicular magnetization film”, Journal of Japan Applied Magnetization, Vol. 24, No. 4-2, 2000, pp. 563–566.
Moreover, as an example of a stacked structure of an MTJ element using perpendicular magnetization films, a stacked structure of magnetic layer (GdFeCo) (50 nm)/interface layer (CoFe)/tunnel barrier film (Al2o3) (2.2 nm)/interface layer (CoFe)/magnetic layer (TbFeCo) (30 nm) is disclosed in “Magnetic tunnel junction device with perpendicular magnetization films for high-density magnetic random access memory” by N. Nisimura, et al., Journal of Applied Physics, Vol. 91, No. 8, Apr. 15, 2002.
As described above, with the conventional MRAMs, it is desirable to both reduce the write current and overcome the thermal agitation property; however, no specific designs therefore have been proposed. Furthermore, with the conventional MRAMs, the write current further increases as the dimensions of the MTJ elements are scaled down; however, no specific means has been proposed which allows the write current to be reduced below about 1 mA in order to increase the capacity of the MRAM.